Not initialising granulepos leads to it having arbitrary values in the
encoded stream including possibly negative values which are not valid
and confuse opusdec.  Explicitly initialise opus_encoder::granulepos
to avoid that problem.

.. and move the plugins to src/encoder/plugins/.

Replacing GLib macros.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces GLib's GError.

I recently opened a bug: http://bugs.musicpd.org/view.php?id=3787

The main problem is that opus encoder config for signal overwrote
bitrate setting.

The "opusinfo" program complained about preskip value that was too
small.  This commit uses OPUS_GET_LOOKAHEAD to obtain the number of
frames that shall be silence at the beginning.

If there are multiple pages, the last partial page must be flushed.